Field of the Invention
The invention relates to a method for producing a holographic optical element wherein a recording stack comprising at least one recording element laminated to at least one supporting element is provided, wherein the recording stack is irradiated by at least one recording beam. The invention relates also to a holographic optical element produced by this method and a display device comprising this holographic optical element.
Discussion of the Related Art
Nowadays, liquid crystal displays are often used in electronic applications. Example applications include mobile devices, game computers, tablet computers, monitors, television devices, advertising panels, and the like. Liquid crystal displays comprise a layer or panel with liquid crystals cells which can be driven electrically. In particular, the polarisation of the light illuminated by the display can be controlled depending on the voltage applied to the liquid crystals cells. Since these panels produce no light of their own a backlight unit has to be provided to illuminate the panels.
Thereby, a general concern is to provide a liquid crystal display having a high display quality. Backlight units which enable a liquid crystal display to fulfil these requirements are backlight units comprising a holographic optical element, in particular, beam shaping holographic optical element. A beam shaping holographic optical element is configured to illuminate a defined area at a defined distance from the beam shaping holographic optical element in a homogenous manner. For instance, the liquid display panel, a lens, a diffusor, or the like may be arranged at the defined area. In particular, by recording an element comprising any suitable recording material with a desired pattern, a beam shaping holographic optical element can be produced. It shall be understood that there are a plurality of further application requiring a beam shaping holographic optical element with good steering qualities, like e.g. signal lighting, projection systems or optical combiners.
In particular, holographic optical elements are the preferred choice for slim optical devices which have to facilitate very complex optical functions, like in e.g. head up (HUD) and head mounted display (HMD) and in autostereoscopic 3D displays (ASD). Especially with the increasing use of light emitting diodes (LED) and laser diodes (LD) as light sources, the advantages of holographic optical elements can be fully exploited.
For producing a holographic optical element, a recording element can be laminated on a supporting element. Then, the recording element can be irradiated by at least one recording beam for a predefined exposure time for generating the holographic optical element having a desired pattern. Generally, it is desired to produce a holographic optical element having a high quality. In particular, it is an objective to produce a holographic optical element with homogeneous diffraction efficiency.
However, a problem of the known recording schemes is that so called fringes can occur. Fringes appear on a holographic optical element as dark and bright rings. In particular, it is difficult to produce large area holographic optical elements without such fringes.
More particularly, fringe formation simply spoils the function and the quality of the holographic optical element. If the holographic optical element is used to be looked into it, for example if it serves as a view box, the fringe is simply visible as dark area in the holographic optical element plane. If the holographic optical element is used to reconstruct for example the real image of a diffusor which could serve as a spatially well defined light source, the fringe simply reduces the diffraction efficiency of the holographic optical element.